Optical systems, amplifiers, or lasers based on large mode area (LMA) fibers may require a good beam quality. The beam quality provided by a light-amplifying optical fiber may be improved by selecting a low numerical aperture (NA), which does not support propagation of high order modes.
It is known that the numerical aperture of a fiber may be reduced by increasing the refractive index of the cladding.
Manufacturing of a large mode area fiber requires tight control of the numerical aperture. A manufacturing process, e.g. core deposition does not always allow accurate control of the refractive index, and there may be variations in the numerical aperture of the resulting fiber. Thus, there may be excessive variations in the numerical aperture of the manufactured fibers. In some cases the numerical aperture may accidentally be too low.
U.S. Pat. No. 6,823,122 discloses light-amplifying fiber which has an inner cladding portion and an outer cladding portion. The refractive index of the inner cladding portion may be reduced e.g. through fluorine and boron doping in order to implement a desired refractive index profile.